As technology in the electronics field advances, circuit densities are perpetually increasing. Discrete components have given way to integrated circuits (IC's) mounted on printed wiring boards. The IC includes a silicon die (chip) mounted in a package which protects and provides I/O (input/output) signal access to the chip. The medium scale integration (MSI) chip has been produced in a variety of different packages (e.g., dual-in-line packages, pin grid array, cerdip, etc.).
Further decreases in silicon die size and the resulting step up to large scale integration (LSI) and very large scale integration (VLSI) have led to the development of hybrid packages (i.e., bare silicon chips mounted on a common ceramic or silicon substrate), TAB (Tape Automated Bonding), ASIC's (Application Specific Integrated Circuits), and surface mount.
State of the art technologies are pushing integration well beyond VLSI. The resultant increase in operational clock speeds (e.g., approaching 10.sup.9 Hz) has resulted in complex new packaging problems. This has led to development of multichip modules (MCM's). The MCM is a wafer-size integration hybrid which includes a plurality of silicon dies mounted on a common substrate. The same process which is used to form the interconnect system on the chip may be used to form the chip-to-chip interconnect on the substrate.
The MCM allows more chips to be brought closer together to accommodate greater I/O rates and greater signal processing speeds. This is accomplished by using fine line interconnection features fabricated through thin film microlithographic techniques.
Pull-up resistors make up a significant percentage of the analog devices on a circuit board. Therefore, it is highly desirable to include them directly on the chip itself in order to further reduce the circuit size. A pull-up resistor or simply a "pull-up" is a simple circuit (e.g., a resistor) which is used to provide a bias voltage to a wire (e.g., a bus) in a digital circuit. Conventionally, the term "pull-up" has referred to a circuit for providing a bias voltage which is equivalent to a logical HIGH (e.g., a positive voltage). The term "pull-down", on the other hand, has conventionally been used to identify a circuit for providing a bias voltage which is equivalent to a logical LOW (e.g., ground or a negative voltage). As used herein, the term "pull-up" is used generically to describe either a pull-up or pull-down circuit unless indicated otherwise.
The pull-up serves a variety of purposes. For example, it may be used to provide a logic HIGH capability to open-drain or open-collector type outputs, to provide higher current sourcing capabilities to totem-pole type outputs, to help control the effects of noise on a bus, and to provide a termination load on a bus (i.e., to provide damping to prevent high frequency ringing).
A pull-up is easily emulated on a chip using a transistor (e.g., an FET) rather than a resistor. It may be connected to an input terminal or an output terminal on the chip. For example, the pull-up may include an N-channel depletion-mode MOSFET with the gate and drain leads connected to the positive voltage supply V.sub.DD and the source lead connected to the I/O terminal to be pulled up. This configuration will cause the transistor to act like a constant current source and to provide V.sub.DD to the I/O terminal. Thus, the general term "pull-up" is not limited to only resistors, but includes any circuit which performs the pull-up function.
A major problem confronted when using on-chip pull-ups is that of inflexibility because a pull-up is not always needed. For example, IC's which are typically connected to a bus (and therefore require pull-ups) are not individually designed or manufactured. Thus, all IC's must either be produced with pull-ups or without. If pull-ups are used on every chip connected to a bus, then the current which a device has to sink in order to pull the bus LOW will be greatly increased and may exceed the limits of the sinking transistor. For example, if a bus has six output drivers connected to it and each driver includes an on-chip pull-up capable of sourcing a nominal 5 mA, then one of the six outputs attempting to pull the bus LOW must be able to sink 30 mA. This current may exceed the current handling capabilities of the sinking output such that it is not capable of pulling the bus LOW or such that the output is damaged from the excess current.
The alternative is to produce two types of each IC chip: one with pull-ups and one without. In this manner, only one of the six chips discussed in the example above could have an on-chip pull-up. This, however, is a cumbersome and expensive approach which is not likely to be commercially accepted.
What is needed is an on-chip pull-up which may be selectively enabled/disabled.